Foamed thermoplastic resin comprising poly(p-methylenebenzoate) has not been previously prepared. Although p-hydroxymethylbenzoic acid, which is the monomer for poly(p-methylenebenzoate) has been known since as early as 1872 when the acid was prepared by free-radical bromination of p-toluic acid to p-bromomethylbenzoic acid, hydrolysis with aqueous barium hydroxide and subsequent purification by recrystallization from water, the low molecular weight of the resulting polymer made the polymer obtained from the acid unsuitable for use in foams. In general, it is known that molecular weight of a polymer must be sufficiently high to cause the resulting polymer to have good cell structure when foamed. Polymers having insufficient molecular weight, or insufficient melt strength, have been demonstrated to not have sufficient cell strength to form bubbles properly, the bubbles tending to deform and tear apart caused by lack of strength to withstand the inflation pressure of the bubble. Melt strength is correlated with measurement of inherent viscosity which, in turn, is used to measure molecular weight. An inherent viscosity (I.V.) of the polymer, which is lower than 0.4 dl/g, as measured in deciliters per gram (dl/g) in a 60/40 phenol/tetrachloroethane solvent at 30.degree. C., is considered too low to allow proper cell formation of foamed poly(p-methylenebenzoate).
Polymers of p-hydroxymethylbenzoic acid having an inherent viscosity of at least 0.44 dl/g have been taught in commonly-assigned U.S. Pat. No. 4,130,719, but foamed thermoplastic resin comprising poly(p-methylenebenzoate) has not been previously prepared because choice of foaming agent had been found to be critical in preparing the foamed composition. Commonly-used foaming agents such as azodicarbonamide have been found to be deleterious when used to form poly(p-methylenebenzoate). Cell coalescence occurs because molecular weight of the polymer apparently decreases.
An object of this invention is to provide a foamed thermoplastic resin of poly(p-methylenebenzoate) having regular structure, good mechanical properties and unique molding characteristics.
A further object of this invention is to provide a process for providing a foamed thermoplastic resin of poly(p-methylenebenzoate) using a foaming agent suitable for the process. The process of thermoplastic structural foam injection molding is well known and adequately described in many sources, notably, J. L. Throne, "Principles of Thermoplastic Structural Foam Molding: A Review," 77-131, in N. P. Suh and N. H. Sung, eds., Science and Technology of Polymer Processing, MIT Press, 1979, which is incorporated by reference, and will not be repeated in detail here.
These and other objects will become apparent upon reading of the specification.